Bright, shiny, metallic finishes are desirable in many commercial applications. The major source of these shiny finishes is chemical or electrical plating of chrome. However, chrome treatments have environmental and economic drawbacks. Chromium is a major source of heavy metal contamination and is expensive to produce. With this in mind, a great deal of attention has been given to producing shiny coatings with powder coating compositions. Powder coatings are generally produced by mixing a binder with other constituents which can be melt mixed, cooled, and pulverized to provide a powder material that is easily applied to a conductive surface. The powder or surface is then heated to cause the powder coating composition to melt and flow to form a continuous coating.
The powder coating compositions are most commonly applied through electrostatic spray coating or fluidized bed coating. In the electrostatic spray process, the coating composition is dispersed in an air stream before being subjected to a high voltage field in which the particles pick up an electrostatic charge. The charged particles are then attracted to a charged substrate resulting in a fine layer of powder on the substrate. The coated substrate is then heated to a temperature sufficient to melt the powder coating and to cause it to flow providing a smooth, even finish.
Fluidized bed coating makes it possible to apply films as thick as about 2.5 mm. In this method, the powder coating composition is fluidized in a fluidized bed by passing a fluidizing gas stream into a vessel containing the powder coating. A substrate is heated in an oven to a temperature above the melting point of the powder coating and is then dipped into the fluidized bed. The powder coating particles contact the hot surface, melt and flow on the substrate surface. See generally, Kirk-Othmer, Concise Encyclopedia of Chemical Technology, pp. 944-945 (1985).
One attempt to achieve a chrome-like finish has been through the incorporation of aluminum particles or aluminum flake into powder coating compositions. However, in our research we have found that the incorporation of aluminum flake into conventional powder coating compositions do not have a shiny chrome-like appearance as measured by the high reflectance of conventional chrome electro-plating.
In light of the current state of the art, a powder coating composition and processes for the production of such powder compositions are needed which can form a glossy, highly reflective metallic finish that appears to be as shiny as chrome plate.